REGULATION OF TRANSPIRATION. 



247 



means of the stomata, in so far that they can be opened and closed. The stomata 

 are, as was mentioned before, the external openings of the intercellular spaces of the 

 parenchyma ; and, according as these openings are closed or open, the exit of the 

 aqueous vapour exhaled from the surfaces of the moist cell-walls in the intercellular 

 spaces is permitted to a greater or less extent. The stomata and their movements 



which depend on irritability, have, as we see, a meaning only in so far as the epidermis 

 lying between them, and provided with cuticle and wax, prevents transpiration 

 in the narrower sense. The aqueous vapour is evidently to be given off not at any 

 haphazard spot of the epidermis, but by the walls of the intercellular spaces; 

 and these are particularly large and numerous in the assimilating parenchyma. 



